Earth Science,
12e
Glaciers, Deserts,
and Wind
Chapter 6
Glaciers: a part of two basic
cycles in the Earth system
Glaciers are a part of both the hydrologic
cycle and rock cycle
Glacier – a thick mass of ice that forms
over land from the compaction and
recrystallization of snow and shows
evidence of past or present flow
Glaciers: a part of two basic
cycles in the Earth system
Types of glaciers
• Valley, or alpine glaciers – form in
mountainous areas
• Ice sheets, or continental glaciers
• Large scale
• e.g., Over Greenland and Antarctica
• Other types
• Ice caps and piedmont glaciers
Currently ice sheets cover
Greenland and Antarctica
Figure 6.2
Glaciers: a part of two basic
cycles in the Earth system
Movement of glacial ice
• Types of glacial movements
• Plastic flow
• Slipping along the ground
• Zone of fracture
• Uppermost 50 meters
• Crevasses form in brittle ice
Glaciers: a part of two basic
cycles in the Earth system
Movement of glacial ice
• Zone of accumulation – the area where a
glacier forms
• Zone of wastage – the area where there is a
net loss due to melting
The glacial budget
Figure 6.7
Glaciers: a part of two basic
cycles in the Earth system
Glaciers erode by
• Plucking – lifting of rock blocks
• Abrasion
• Rock flour (pulverized rock)
• Striations (grooves in the bedrock)
Glaciers: a part of two basic
cycles in the Earth system
Landforms created by glacial erosion
•
•
•
•
•
•
Glacial trough
Hanging valley
Cirque
Arête
Horn
Fiord
Erosional landforms created
by alpine glaciers
Figure 6.10 C
The Matterhorn in
the Swiss Alps
Figure 6.12
Glaciers: a part of two basic
cycles in the Earth system
Glacial deposits
• Glacial drift
• All sediments of glacial origin
• Types of glacial drift
• Till – material that is deposited directly by ice
• Stratified drift – sediment deposited by
meltwater
Glacial till is typically
unstratified and unsorted
Figure 6.13
Close-up view of the boulder
in the previous slide
Glaciers: a part of two basic
cycles in the Earth system
Glacial deposits
• Depositional features
• Moraines – layers or ridges of till
• Types of moraines
• Lateral
• Medial
• End
• Ground
Glaciers: a part of two basic
cycles in the Earth system
Glacial deposits
• Depositional features
•
•
•
•
•
Outwash plain, or valley train
Kettles
Drumlins
Eskers
Kames
Glacial depositional
features
Figure 6.18
Kettle lakes
Figure 6.18
Glaciers: a part of two basic
cycles in the Earth system
Glaciers of the past
• Ice Age
• Began 2 to 3 million years ago
• Division of geological time is called the Pleistocene
epoch
• Ice covered 30% of Earth’s land area
Maximum extent of ice
during the Ice Age
Figure 6.22
Glaciers: a part of two basic
cycles in the Earth system
Glaciers of the past
• Indirect effects of Ice Age glaciers
•
•
•
•
Migration of animals and plants
Rebounding upward of the crust
Worldwide change in sea level
Climatic changes
Glaciers: a part of two basic
cycles in the Earth system
Causes of glaciation
• Successful theory must account for
• Cooling of Earth, as well as
• Short-term climatic changes
• Proposed possible causes
• Plate tectonics
• Continents were arranged differently
• Changes in oceanic circulation
Glaciers: a part of two basic
cycles in the Earth system
Causes of glaciation
• Proposed possible causes
• Variations in Earth’s orbit
• Milankovitch hypothesis
• Shape (eccentricity) of Earth’s orbit varies
• Angle of Earth’s axis (obliquity) changes
• Axis wobbles (precession)
• Changes in climate over the past several
hundred thousand years are closely associated
with variations in Earth’s orbit
Deserts
Geologic processes in arid climates
• Weathering
• Not as effective as in humid regions
• Mechanical weathering forms unaltered rock and
mineral fragments
• Some chemical weathering does occur
• Clay forms
• Thin soil forms
Deserts
Geologic processes in arid climates
• Role of water in arid climates
• Streams are dry most of the time
• Desert streams are said to be ephemeral
• Flow only during periods of rainfall
• Different names are used for desert streams
including wash, arroyo, wadi, donga, and nullah
Deserts
Geologic processes in arid climates
• Role of water in arid climates
• Desert rainfall
• Rain often occurs as heavy showers
• Causes flash floods
• Poorly integrated drainage
• Most erosional work in a desert is done by running
water
A dry stream channel
in the desert
Figure 6.28 A
The same stream channel
following heavy rainfall
Figure 6.28 B
Deserts
Basin and Range: the evolution of a desert
landscape
• Uplifted crustal blocks
• Interior drainage into basins produces
• Alluvial fans and bajadas
• Playas and playa lakes
Deserts
Basin and Range: the evolution of a desert
landscape
• Erosion of mountain mass causes local relief
to continually diminish
• Eventually mountains are reduced to a few
large bedrock knobs called inselbergs
projecting above a sediment-filled basin
Landscape evolution in a
mountainous desert – early
Figure 6.29 A
Landscape evolution in a
mountainous desert – middle
Figure 6.29 B
Landscape evolution in a
mountainous desert – late
Figure 6.29 C
Deserts
Wind erosion
• Deflation
• Lifting of loose material
• Produces
• Blowouts
• Desert pavement
• Abrasion
Formation of
desert pavement
Figure 6.34
Deserts
Types of wind deposits
• Loess
• Deposits of windblown silt
• Extensive blanket deposits
• Primary sources are deserts and glacial stratified
drift
Deserts
Types of wind deposits
• Sand dunes
• Mounds and ridges of sand formed from the wind’s
bed load
• Characteristic features
• Slip face – the leeward slope of the dune
• Cross beds – sloping layers of sand in the dune
Formation of sand dunes
Figure 6.37
Deserts
Types of wind deposits
• Sand dunes
• Types of sand dunes
• Barchan dunes
• Transverse dunes
• Longitudinal dunes
• Parabolic dunes
• Star dunes
Sand dune types
Figure 6.38
Sand dunes in the
western United States